Motion picture films comprise silver-halide crystals dispersed in an emulsion, which is coated in thin layers on a film base. The exposure and development of these crystals form the photographic image consisting of discrete tiny particles of silver. In color negatives, tiny blobs of dye occur on the sites where the silver crystals form following chemical removal of the silver during development of the film stock. These small specks of dye commonly bear the label ‘grain’ in color film. Grain appears randomly distributed on the resulting image because of the random formation of silver crystals on the original emulsion. Within a uniformly exposed area, some crystals develop after exposure while others do not.
Grain varies in size and shape. The faster the film, the larger the clumps of silver formed and blobs of dye generated, and the more they tend to group together in random patterns. The term “granularity” typically refers to the grain pattern. The naked eye cannot distinguish individual grains, which vary from 0.0002 mm to about 0.002 mm. Instead, the eye resolves groups of grains, referred to as blobs. A viewer identifies these groups of blobs as film grain. As the image resolution becomes larger, the perception of the film grain becomes higher. Film grain becomes clearly noticeable on cinema and high-definition images, whereas film grain progressively loses importance in Standard Definition Television (SDTV) and becomes imperceptible in smaller formats.
Motion picture film typically contains image-dependent noise resulting either from the physical process of exposure and development of the photographic film or from the subsequent editing of the images. Photographic film possesses a characteristic quasi-random pattern, or texture, resulting from physical granularity of the photographic emulsion. Alternatively, simulation of similar pattern can occur in computed-generated images in order to blend them with photographic film. In both cases, this image-dependent noise bears the designation of “film grain.” Quite often, moderate grain texture presents a desirable feature in motion pictures. In some instances, the film grain provides visual cues that facilitate the correct perception of two-dimensional pictures. Film grade often varies within a single film to provide various clues as to time reference, point of view, etc. Many other technical and artistic demands exist for controlling grain texture in the motion picture industry. Therefore, preserving the grainy appearance of images throughout image processing and delivery chain has become a requirement in the motion picture industry.
Several commercially available products have the capability of simulating film grain, often for blending a computer-generated object into natural scene. Cineon® from Eastman Kodak Co, Rochester N.Y., one of the first digital film applications to implement grain simulation, produces very realistic results for many grain types. However, the Cineon® application does not yield good performance for many high speed films because of the noticeable diagonal stripes the application produces for high grain size settings. Further, the Cineon® application fails to simulate grain with adequate fidelity when images become subject to prior processing, for example, such as when the images are copied or digitally processed.
Another commercial product that simulates film grain is Grain Surgery™ from Visual Infinity Inc., which is used as a plug-in of Adobe® After Effects®. The Grain Surgery™ product appears to generate synthetic grain by filtering a set of random numbers. This approach suffers from disadvantage of a high computational complexity.
Film grain simulation continues to remain a topic of interest. Recently, the H.264 Video Compression standard underwent amendment to adopt a standard Supplemental Enhancement Information (SEI) message specifying film grain simulation parameters. The adoption of a specific format film grain SEI message will greatly facilitate the development of decoders for simulating film grain using a technique recently disclosed by the assignee of the instant application for simulating film grain. The technique developed by applicants' assignee simulates film grain by (1) first filtering grain out of images before compression, then (2) transmitting compressed video together with a message containing information about original grain to a decoder, and (3) restoring the original grainy appearance of images by having the decoder simulating film grain based on the content of the SEI message. This technique provides large bit-rate savings for high quality applications requiring the film grain preservation. However, the technique increases decoder complexity since film grain generation requires the creation of the film grain image by operating on blocks of 16×16 pixels, and subsequent deblocking of 16×16 block edges of the film grain image.
Thus, a need exists for a film grain simulation technique that overcomes the prior disadvantages by providing greater efficiency.